Ros Altmann, a pensions expert and government adviser, said: “It is simply not good enough for banks to keep customers in the dark about rate changes.”

She added: “At the very least, financial institutions should let people know on a monthly basis if their interest rates change.”

The interest rates available for a whole range of savings products, covering everything from current accounts to ISAs, have tumbled for much of the past year as the Government and Bank of England flooded the financial services sector with cheap funding through schemes such as quantitative easing. SavingsChampion, an interest rate monitoring website, said West Bromwich Building Society had made a total of 75 cuts across its wide range of easy access savings accounts since the start of the year.

Over the same period, Sainsbury’s Bank made 38 changes and Cambridge Building Society 32. Last year there were just 86 across the whole 12 months.

Royal Bank of Scotland, which this week posted profits of £1.4 billion, insisted it always wrote to customers “whenever we make a change, up or down”.

A spokesman for the Financial Conduct Authority, which replaced the Financial Services Authority in April, said: “As the regulator we will be looking extremely closely to see whether or not it’s fair to their customers to reduce rates without notifying them.”